Chapter 6. Peanut Disease Management

Wise management of diseases is an essential part of profitable peanut production. Long rotations, use of resistant varieties, early disease detection, weather-based disease prediction, scouting, and proper pesticide selection comprise the basic elements of a disease management strategy.

The best way to manage plant diseases is to avoid them by using long rotations. Reductions in peanut acreage in North Carolina have given the opportunity to grow crops that do not host peanut diseases. Fields with a history of disease should be planted with varieties resistant to those diseases. Early detection is critical in developing effective and low cost approaches to reduce the potential impact of diseases. Thorough scouting for and accurate identification of diseases is critical. Anticipation of disease outbreaks by using weather-based predictive models is key to minimizing unnecessary pesticide use and adding precision to those applied. When diseases do occur, correct identification is necessary to evaluate the need for and types of pesticides to apply. After the season is over, maps should be drawn showing where soil-borne diseases (CBR, Sclerotina blight, pod rot, nematodes) were located. These maps will serve as guides for future rotations and use of resistant varieties.

Growers should be aware that information can be found on the Web to help identify disease problems:

Cultural and chemical controls are usually used in combination for maximum benefit. Cultural control methods, such as rotation and resistant varieties, generally reduce the number of disease-causing organisms (pathogens), such as nematodes and fungi. Pesticides are only useful when cultural practices have not sufficiently reduced pathogen levels below the threshold.

This chapter covers disease identification and management. Important tables list best rotational crops (Table 6-1), effect of tillage practices (Table 6-2), disease resistant varieties (Table 6-3), a comparison of CBR and Tomato Spotted Wilt Virus (TSWV) symptoms (Table 6-4), and chemical controls for peanut diseases (Table 6-5). A disease management calendar (Table 6-6) organizes disease management issues to be considered from the start to the end of the season. The following information describes each major disease of peanut, its cause, and management considerations unique to each disease.
 
 

Foliar Diseases

Peanut leaf spot is caused by two different fungi: Cercospora arachidicola (early leaf spot pathogen) and Cercosporidium personatum (late leaf spot pathogen). There are other important leaf diseases that cause spots, but are not referred to as "leaf spot" (see other diseases listed below). It is often difficult to distinguish between these two diseases, particularly since symptoms depend on the variety grown and the fact that lesions change in appearance as the season progresses. Early leaf spot has light-brown lesions generally surrounded by a yellow halo and can be found as early as June 1. Late leaf spot has darker spots, usually without a halo, that appear later in the season. The best way to tell these two diseases apart is by using a good magnifying glass to see where spores are being produced. Early leaf spot spores are silvery and hairlike and appear on the spot on the top of the leaf. Late leaf spots are brown and velvety and are formed mostly on spots on the bottom of the leaf. It is important to determine if late leaf spot becomes predominate since some fungicides are not very effective against this disease.

Use long rotations (Table 6-1) with any crop other than peanut, and varieties with resistance to this disease (Table 6-3). Avoid highly susceptible varieties. Conservation tillage appears to reduce leaf spot slightly (Table 6-2). Also see chemical controls (Table 6-5).

Fungicides can be applied in one of two ways: 1) a set calendar 14-day schedule, or 2) a weather-based leaf spot advisory schedule. The advisory is a safe method of minimizing the unnecessary use of fungicides by spraying only when weather conditions favorable for disease have occurred. For more information on weather-based advisories contact your county agent and read the section below on Disease Forecasting Advisories.

Web blotch (Phoma arachidicola) is a sporadic problem that was very serious in the 2000 and 2001 growing seasons. A large (1/2 inch), circular, dark area forms on the upper surface of the leaf, which can dry and crack as it ages. Severe defoliation often occurs in portions of a field, but can spread over the entire field in a short time.

Use long rotations (Table 6-1) with any crop other than peanut, and varieties with resistance to this disease (Table 6-3). Avoid highly susceptible varieties. Also see chemical controls (Table 6-5). Propiconazole 3.6EC + trifloxystrolin 50WG (Stratego) and Chlorothalonil (various formulations) have activity against early leaf spot, late leaf spot, and web blotch. Azoxystrobin (Abound) 2.08F, tebuconazole 3.6F (Folicur) and propiconazole 45WP + flutolanil 50WP (Montero) control early and late leaf spot, web blotch, stem rot and rhizoctonia limb rot (Table 6-3). Frequent applications of chlorothalonil are the most effective means of controlling this disease.

Pepper spot (Leptosphaerulina crassiasca) is present every year in all fields as very small dark lesions that "pepper" a leaf. Occasionally, lesions will fuse and kill large areas, resulting in a scorch symptom. Pepper spot has been associated with severe vine decline, which sometimes occurs after a heavy late-season rain. Control is the same as for leaf spot.

Botrytis blight (Botrytis cinerea) is most commonly seen at the end of the season when conditions are moist. Symptoms appear first on vines or leaves that have been injured (i.e. tractor tires or freezing temperatures) killing the tissue. Massive numbers of brown spores can be produced and black sclerotia can be found on the pods of peanuts. While this disease does not usually cause serious losses, it can be alarming.

Symptoms are sometimes seen on the leaves without occurring on any other part of the plant. Numerous lesions can appear on the top of the leaf. Spots are about the same size as early leaf spot lesions; however, they are light tan in color with no obvious spores.

Timely harvest and avoidance of plant injury will reduce symptom incidence and severity. Benomyl (Benlate SP) 50 WP at 4 ounces per acre can be used as a chemical treatment.

Phytotoxicity. Phytotoxicity (chemical toxicity) caused by systemic insecticides applied at planting is often confused with leaf spot. These symptoms will usually be around the margins of the leaflets on the lower most leaves. In general, spots found before mid-June are phytotoxicity. Herbicide toxicity can also cause leaf spots by burning areas associated with the spray droplets. Symptoms associated with phytotoxicity will not spread or form spores.

Learn to identify symptoms associated with chemical damage to keep from mistaking symptoms for a contagious problem such as leaf spot. Avoid practices that lead to symptoms.

Tomato spotted wilt virus (TSWV) is found in all peanut counties in North Carolina. Spread is accomplished primarily through thrips, which obtain the virus by feeding on infected plants (cultivated or wild plant species). Symptoms vary but usually include one or more of the following: stunting, dead terminal buds, pale yellow or white ring patterns on leaves; purple blotches on the underside of leaves; stunted, small, and malformed growth; undersized pods; and red seed coats. On some occasions, the whole plant may turn light green, resembling CBR disease (Table 6-4). Clumps of two or three symptomatic plants are usually found scattered throughout a field.

Rotation is not effective in managing TSWV. There are considerable differences in the level of resistance of different varieties. Severity can be reduced by planting resistant varieties (Table 6-3). Poor plant stands (less than 4 plants per foot of row) tend to be more severely affected by this disease. In a like manner, twin rows have less spotted wilt than single row planting patterns. Thick planting and twin rows tend to have less disease. Minimum tillage appears to reduce the incidence of TSWV while conventional tillage seems to make the disease somewhat worse (Table 6-2). In-furrow use of phorate or Thimet seems to reduce disease incidence more so than Temik. Over the top insecticides targeting thrips do not appear to reduce the disease. Some observations have been made that imply that there is somewhat less TWSV when Thimet and Karate are both applied. These observations have not been confirmed.

In 2001, Steve Barnes, Superintendent at the Peanut Belt Research Station in Lewiston, N.C., noticed that peanuts grown in an experimental area receiving chicken litter had considerably more TSWV. Investigations will begin in 2002 to examine this issue.

Research conducted on seed transmission, thus far, have shown no evidence that this occurs. Symptomatic seed planted in the field have not resulted in plants with TSWV. Further studies are planned on this issue as many growers are concerned about this possibility.

TWSV has gotten worse since it was first observed in North Carolina about 8 years ago. The pattern of disease increase has closely paralleled that which occurred in Georgia starting several years earlier. Georgia peanuts have experienced a decline in the amount of TSWV over the last 3 years. There is reason to think that North Carolina may begin to see a natural decline in the disease over the next 5 years, if the patterns observed in the past hold true.

While most soil-borne pathogens can survive for years in the soil, their numbers decrease each season when there are no plants to infect. Very long rotations allow numbers of these organisms to eventually reach low levels so that little or no plant disease or loss occurs. These organisms do not travel far, consequently, mapping the location and intensity of soil-borne diseases is a useful tool for deciding where certain cultural practices and/or chemical treatments will be applied next time peanuts are grown. Soil-borne diseases are caused by nematodes and fungi.

Nematodes

Peanut nematodes include the Northern root knot (Meloidogyne hapla), peanut root knot (Meloidogyne arenaria), lesion (Pratylenchus brachyurus), ring (Criconemella ornata), and sting (Belonolaimus longicaudatus) nematodes.

Nematodes cause plants to be stunted, wilted, or off-colored. Nematode damage can also increase susceptibility to black root rot (CBR). Fields to be planted to peanuts should be sampled for nematode populations in the previous fall (September through November). Twenty probes (1 inch in diameter to an 8-inch depth in the row) should be taken for each sample, one sample to each 4 or 5 acres. Samples will be more representative of the field if the soil is mixed by disking before samples are collected.

Samples should be taken in a zigzag pattern across the field. Divide fields requiring more than one sample according to the row direction so only infested areas receive a nematicide treatment. To prevent decomposition, nematode samples should remain cool (50-60 degrees) and be given to the county Extension agent or sent to the North Carolina Department of Agriculture Nematode Advisory Service as soon as possible after collection.

Fields that are below threshold levels (A category) need no control procedures. Fields that are B category are borderline cases; treatment may give a return on control investment, but probably will not. C category fields are above threshold levels and should receive an appropriate control.

Root knot nematode damage on the roots and pods may show up either as small round lesions about the size of the head of a pin (Meloidogyne hapla) or large swellings or galls (Meloidogyne arenaria). Checking pods or roots for galls after digging may indicate where a root knot problem may be found.

Nematodes are often found in spots or small areas of fields. For the best use of nematicide dollars, treat for nematodes on a field-by-field or part of a field basis rather than treating the entire crop. Apply nematicides only according to label directions for injection, banding, and/or incorporation. Applying of Temik in the furrow at planting for foliar insect control may help in controlling nematodes but does not replace banded or injected nematicide treatments when C category nematodes are found. Never leave granular nematicides/insecticides on the soil surface. Always incorporate, particularly at the row ends, to avoid bird kills.

Planting crops that do not support the growth and reproduction of nematodes reduces their numbers (Table 6-1). Long rotations are the most effective method of controlling nematodes and can be used instead of nematicides. Conservation tillage appears to reduce nematode numbers slightly (Table 6-2).

Seed and seedling rots can be caused by many fungi. Seeds will either not germinate (seed rot), germinate but not penetrate the soil surface (pre-emergence damping off), or die shortly after emergence (post-emergence damping off). The result is a poor stand with skips which can lead to more TSWV. Often the primary problem is either environmental (poor seedbed conditions) or poor seed quality, rather than disease per se.

Planting should be done in warm soil (65 degrees at a 4-inch depth for 3 consecutive days), since cold soils retard germination and increase the chance for rots. Bedding generally aids soil warming and drainage. Poor drainage can cause waterlogging, a major factor in seed and seedling rots. Seed and seedling diseases can usually be prevented by using high-quality seed coated with a good chemical seed treatment fungicide.

After plant emergence, count several 100-foot lengths of row to determine the number of healthy seedlings. The goal is to determine if sufficient plants are present to make a good crop and minimize TSWV. Up to mid-May, an average of 400 plants or more per 100 feet of row is usually sufficient. If plant populations are below these levels, the only recourse is to replant.

Southern stem rot (southern stem rot or white mold) (Sclerotium rolfsii) is found in all peanut counties of North Carolina. It is a well-known disease, but does not commonly cause large yield losses. Limbs with southern stem rot normally have white, stringy fungus growth and yellow to brown birdshot-sized balls (sclerotia) on the lower stems, and leaf litter. These sclerotia distinguish southern stem rot from other soil-borne diseases. This disease is most active during the hottest part of the season, especially following rain, causing wilting and sometimes death. This disease is often found along with CBR. If these diseases have been serious problems in a field, then peanuts should be rotated longer with non-host crops (Table 6-3). Fields with heavy vine growth and excess moisture are most prone to this disease.

Contrary to popular belief, conservation tillage may actually reduce disease incidence (Table 6-2). NC10C and several runner varieties are moderately resistant to this disease. Most varieties are susceptible (Table 6-3) to this disease. Fungicides listed for disease control are listed in Table 6-5. Most soil fungicides work best when put out just prior to disease onset. Treat fields with a history of problems according to the leaf spot advisory between July 15 and the end of August. Unusually warm and wet weather makes this disease more severe. Irrigated fields are most severely attacked.

Rhizoctonia limb and pod rot (Rhizoctonia spp.) is sometimes confused with southern stem rot. While both affect the stems, rhizoctonia limb rot does not produce the white stringy growth or sclerotia. Lesions are usually found on the bottom (touching the soil) of stems. Dead areas have a purple border around the diseased area. Rhizoctonia often causes a serious pod rot, the most destructive phase of the disease. Rhizoctonia limb and pod rot is most common in moist fields or where the vines are thick. Irrigated fields are most severely attacked. Management practices are the same as southern stem rot (see above).

Sclerotinia blight (Sclerotinia minor) is found throughout North Carolina, but it is most severe in the most northerly counties. This disease starts by killing individual limbs rather than causing an overall wilt. Careful scouting is required to see this disease when symptoms first appear. Vines must be pulled back to expose lower stems and early infection. The end portion of infected limbs will remain green and look healthy for some time. Only after the disease has been present for many days will limbs be visibly wilted. Sclerotinia blight exhibits cottony mold growth (seen on humid mornings) on straw-colored stem injuries (lesions). Small black sclerotia (irregular in shape) can be seen both on and in infected tissues.

Rotate as long as possible with cotton or corn (Table 6-1). Avoid vine injury by cultivating early (before June 15) or not at all, using small tires, and spraying for leaf spot on the advisory program to minimize trips across the field. Conservation tillage appears to be somewhat beneficial in controlling this disease (Table 6-2). Several varieties have some resistance (Table 6-3). Avoid susceptible varieties. Fields planted early (by May 1) tend to have less Sclerotinia blight. This also allows earlier harvest, which minimizes late-season damage.

Fields with a history of serious problems should be treated with fungicide (Table 6-3) when vines are close to touching, and Sclerotinia blight is first observed or according to the weather-based Sclerotinia blight advisory. First spray applications are usually made after mid-July and sometimes as late as September. Apply more fungicide if the disease is continuing to spread 3 to 4 weeks after the last application.

A number of weeds have now been shown to be hosts of Sclerotinia minor, the fungus that causes Sclerotinia blight. Once infected they allow reproduction of the fungus, potentially resulting in more disease on peanut. Weed hosts include, yellow nutsedge, henbit, small flowered bittercress, common chickweed, mouse-ear chickweed, swinecress, cutleaf evening primrose, horseweed, wild mustard, and mouse-ear cress. It recommended that fields with a history of Sclerotinia blight be kept clean of these weeds.

CBR (Cylindrocladium black rot or black root rot) (Cylindrocladium parasiticum) is found in all peanut counties in North Carolina. Plants become light green and die as a whole, although some limbs may die before others. A blackened, rotting root system, which allows plants to be easily pulled from the ground, is characteristic of this disease. Red fungus structures (as large as the head of a pin) may be found on dead tissue near the ground following moist weather. CBR can be confused with tomato spotted wilt virus (TSWV) late in the season when both cause root rots (Table 6-4).

Long rotations help to reduce the amount of fungus in the soil (Table 6-1). Non-hosts, such as cotton, corn, sorghum, and small grains, are excellent rotations and will help reduce Cylindrocladium populations in the soil. Peanut following peanut or soybean is a formula for disaster and will quickly lead to heavy infestations once disease gets started. Tillage practices don’t significantly affect CBR (Table 6-2) except that small grain mulches appear to reduce disease. Also, no-till appears to reduce the soil-borne inoculum of this disease that could ultimately lead to less disease. An important key in CBR management is the use of resistant varieties (Table 6-3). CBR can be seed transmitted; however, new procedures being used by the seed industry should reduce this already low transmission rate to an insignificant level.

Fields with a history of CBR should be planted on a bed around mid-May to maximize soil warmth. CBR infects during cool, wet periods in the spring and in the fall. Fall infections cause less damage than spring infections. Mid-summer symptoms are usually the result of a spring infection.

If CBR has been identified in a field, submit a nematode sample the fall before peanuts are to be planted. Root knot nematode makes the disease worse on all peanuts. Ring nematode affects all but resistant varieties in the same way. It is not clear what effect the other nematodes may have. Use the nematode assay information to aid in determining which chemical treatment is most suitable.

Soil fumigation with metam sodium (Table 6-5) and planting a resistant variety usually controls CBR. Both fumigation and a resistant variety are needed when peanut fields have a history of 10 percent or more disease incidence in prior peanut crops. Fields with a history of 1 to 10 percent disease should be planted to a resistant variety. Fumigants must be injected 12 inches deep (below the top of the bed) at least 2 weeks prior to planting to prevent damage to young peanut plants. If cool and/or wet conditions are prevalent following fumigation, growers may want to use a lettuce seed bioassay to assure that it is safe to plant. Incorporation of herbicides prior to bedding and injecting gives adequate weed control. There is some evidence that tebuconazole (Folicur) may reduce the symptoms of CBR. However, research has demonstrated that Folicur can adversely affect stand establishment when applied in the seed furrow.

Addition of nitrogen can often reduce yield losses. The form of nitrogen does not seem to be important. Thirty pounds of actual nitrogen (i.e., equivalent to 180 pounds of ammonium nitrate) applied in early June and again in early July has given the best results. This treatment is not recommended as a general cultural practice, rather, as a pre-symptom treatment in fields with a history of CBR. This treatment is not a substitute for using a resistant variety or for fumigating. Responses vary and are often negligible.

CBR Seed Transmission

Another concern associated with the spread of CBR is seed transmission. To develop a better understanding of the significance of seed transmission of CBR, it is first important to emphasize that under most circumstances, there is little or no seed transmission. That is true if: 1) the seed are produced in clean fields, 2) infested seed are sorted out at the shelling plant, 3) the fungus dies during drying, storage, or following seed treatment, or 4) disease is not transmitted due to unfavorable field conditions.

Seed transmission of CBR has become an issue because at times it does occur. A survey of 62 commercial seed lots sampled after conditioning, but before the application of seed treatment, showed that an average of 1.3 percent (range of 0 to 5.6 percent) of the seed had evidence of Cylindrocladium parasiticum colonization (i.e. cinnamon colored speckles about the size of a pencil dot). The speckles are the resting structures (sclerotia) of the fungus. Speckled seed were found in at least one subsample of every seed lot. While all speckled seed had viable C. parasiticum at harvest, by planting time only 4 to 45 percent of speckled seed had viable the fungus. This means that most of sclerotia had died during the winter storage. When the fungus remains alive, these seed can serve as a source of infection to plants emerging from the speckled seed and nearby uninfested plants.

To avoid seed transmission of peanut diseases, peanuts produced for seed should be grown in fields with little or no CBR or Sclerotinia blight (which is also suspected of being seed transmitted). Fields should be on long rotations, fumigated prior to planting, and scouted late in the season to identify these diseases. Portions of fields heavily infested with either of these diseases should be harvested separately and used as edible peanuts. No other disease is known to be transmitted on seed at levels high enough to cause concern. There is good evidence that tomato spotted wilt virus (TSWV) is not seed transmitted.

Methods to Reduce/Eliminate CBR

North Carolina Crop Improvement Association supports the following recommendations that will reduce/eliminate seed transmission of Cylindrocladium parasiticum (CBR).

Plants grown in soils infested with the fungus that causes CBR can produce seed colonized with this fungus. Seed have visible fungus on the seed coat seen as cinnamon colored speckles. Planting speckled seed can result in infesting soils with the CBR fungus.

Recommendations for Certified Seed Production

• Maintain accurate records of field history and maps of CBR incidence.

• Adopt a minimum of 3-year rotations of peanut with non-hosts of Cylindrocladium parasiticum (cotton, corn, etc.).

• Select fields with low levels of CBR.

• Fumigate fields with metam after soil temperatures are less than 65° F at 4 in. depth as reported by weather stations, www.ipm.vt.edu/infonet/, or by contacting your Cooperative Extension center.

• Consult weather forecast (www.wunderground.com or www.weather.com) and delay fumigation when heavy rainfall saturates soil or cold weather is likely to reduce soil temperature below 65° F.

• Inspect fields at the end of the season.

• Selectively harvest infested fields to avoid heavily infested areas.
• Avoid harvesting seed peanuts where disease exceeds 5 percent disease incidence.

Field scouts should be careful not to confuse CBR with Tomato Spotted Wilt Virus late in the season as symptoms can be confusing (see http://www.ces.ncsu.edu/depts/pp/notes/Peanut/gallery/index.html).

Pod rot (many fungi) can be a difficult disease to control because the causes are so diverse. Symptoms include darkening and/or rotting of the pods. Pod rot can be caused by all of the soil-borne diseases listed above. In addition, Pythium spp. are often associated with pod rots. Some pod rot symptoms can be the result of poor calcium nutrition or excessive magnesium or potash levels which weaken the hull, allowing various soil fungi to grow into and rot the pod. Plants grown in soils with proper fertility can also have severe pod rot if pathogen numbers are present in high numbers or insect damage is severe. High pathogen numbers are usually the result of very short rotations. There is evidence that there is less pod rot in no-till fields.

Long rotations (with cotton or corn) usually have the very positive effect of reducing the numbers of pod rotting organisms (Table 6-1). Tillage practices do not seems to have a significant impact on pod rot (Table 6-2). Addition of landplaster has also been shown to reduce the likelihood of some pod rots. The fungicides tebuconaole (Folicur), flutolanil (Moncut or as a component of Montero), and azoxystrobin (Abound) are able to reduce damage caused by the pod rot phase of stem rot and rhizoctonia limb rot. They have no effect on pod rot caused by pythium or sclerotinia.

Ridomil plus PCNB (Ridomil PC) has broad spectrum activity against several pod rotting organisms, including pythium (Table 6-5). However, it is not as effective against the most important pod rotting organism, rhizoctonia, as Folicur and Moncut. All soil-borne diseases (i.e., stem rot, rhizoctonia, sclerotinia, CBR, etc.) can also cause pod rot. Therefore, soil-borne disease management is also beneficial in reducing pod rot. The most economical plan for pod rot control is to rotate as long as possible and use a fungicide if necessary.

A Few Words About Chicken Litter. Recycling nutrients is often a good idea, however, chicken litter has the potential to add toxic levels of zinc to peanut soils and may also make Tomato Spotted Wilt Virus (TSWV) worse. As far as I know, zinc and TSWV are unrelated issues. However, it has been observed that both problems may be worse after repeated use of chicken litter.

We know that peanuts are very susceptible to zinc. Typically, areas where tin roofed sheds stood for years have a patch of poor growth in peanut fields. This is due to the cumulative release of zinc from the tin. Zinc is added to feed as a nutritional supplement for chickens, and is found in abundance in chicken litter. Once spread in soil, this non-mobile metal is toxic to peanuts. Once present at toxic levels (i.e., index of 200 to 400) it is likely to damage peanuts for years to come. Stunting and characteristic split stems are indicators of this problem. It appears that this problem has been growing worse over the years.

In 2001, Steve Barnes, Superintendent at the Peanut Belt Research Station in Lewiston, N.C., noticed that peanuts grown in an experimental area receiving chicken litter had considerably more TSWV. Investigations will begin in 2002 to examine this issue (also see the section in chapter on TSWV).

Digging Date to Reduce Losses to Late Season Diseases is Seldom Effective. Growers often use early digging as a disease management tool for Sclerotinia blight, southern stem rot, TSWV, and CBR. In general, early digging as a method of minimizing soil-borne disease losses is a mistake. In general, healthy peanuts gain in weight and marketability more rapidly than diseased ones are lost. As a rule of thumb, harvesting early is a losing proposition until there is at least 50 percent disease. If more than 50 percent of the plants are diseased, early digging may be advisable. Most soil-borne diseases are not evenly distributed across the field. Therefore, if a grower decides to dig early it is advisable to dig the most diseased portion of a field early and the remainder at the "normal" harvest time.

Adjuvants. Adjuvants are sold as a means to enhance the effectiveness of a pesticide. Fungicides for peanut disease control do not normally need adjuvants. Many claims are made for adjuvants. Consumers can check out some of the claims by asking for the California registration number and label. Data on efficacy is available where the products are registered with California.